A nuclear logging technology is one of booming cutting-edge logging technologies with the development of contemporary nuclear technologies and the demand of petroleum, coal, geological mineral, or the like for the nuclear logging technology. From the natural gamma ray logging for measuring formation natural radioactivity to the natural gamma ray spectrum logging for calculating contents of uranium, thorium and potassium, to geochemical logging, and then to the current elemental capture spectroscopy (simply referred as ECS) for measuring a thermal neutron capture spectrum of a formation element. The nuclear logging technology provides an effective solution for identifying lithology calculating the skeleton density, determining reservoir physical parameters, researching on sedimentary environment and stratigraphic correlation in a complex reservoir, or the like.
The kernel of the elemental capture spectrum logging interpretation is as below. Firstly, an original measurement spectrum of the formation element is graduated by using a standard spectrum of each element, to obtain a yield of each element by spectrum unfolding. Secondly, the yield of each element is converted to a weight percentage content of each element constituting a formation rock based on an “oxygen closed” theory. Finally, the weight percentage content of the element is converted to that of each mineral by establishing a transformational relation between the formation element and the formation mineral, to implement lithology identification and rock skeleton parameters calculation. However, the key of the elemental capture spectrum logging is how to accurately obtain the standard capture gamma ray spectrum of each element under the simulated logging condition in laboratory, thereby graduating the original measurement spectrum of the stratum, which is the foundation of a complete processing interpretation in elemental capture spectrum logging.
However, in the prior art, there only exists a method for measuring a standard capture gamma ray spectrum of single element such as silicon, calcium, iron, titanium, or the like, and there is a great difference between these spectral lines and the standard data provided by a nuclear database of the international atomic energy agency (IAEA).